Human granulocytic ehrlichiosis (HGE) is a newly recognized, tick-borne infection. The HGE agent colonizes neutrophils, resulting in fever, leukopenia, and thrombocytopenia. The HGE agent can be propagated in promyelocytic HL-60 cells, facilitating in vitro pathogenesis studies. Preliminary studies in our laboratory suggest that during residence in HL-60 cells, the HGE agent prevents the respiratory burst by downregulating transcription of gp91phox, an integral component of NADPH oxidase, by a currently undefined mechanism. Deciphering how the HGE agent inhibits gp91phox expression and identifying other host cell genes that are differentially expressed during infection are crucial to the rational design of therapies and vaccines. Therefore, transcriptional studies using retinoic acid-induced HL-60 cells, metamyelocytic PLB-985 cells, and a murine model of granulocytic ehrlichiosis will be performed to determine if gp91phox expression is inhibited by the HGE agent during residence in differentiated myeloid cells and mammalian infection, respectively. RT-PCR and immunoblot analyses will assess whether the HGE agent blocks gp91phox transcription by inhibiting expression of gp91phox transcriptional regulators. Electrophoretic mobility shift assays will define whether binding of the gp91phox transcriptional regulators is inhibited by the HGE agent. These studies, in addition to providing a greater understanding of HGE, may also offer clues as to the intracellular survival mechanisms of other microbial pathogens.